Frame Jig design - how much adjustment is really needed for home project?
I'm in the process of designing a frame jig for a bit of a weekend project that I'm hoping to do and I need some advice from some more experienced builders. Looked at many different designs from commercial heavy duty metal monsters all the way to DIY timber one use efforts.
The more adjustments I allow my jig to make, the more cost and difficulty enter into it - or it is just going to be hard to use and inaccurate.
I've been studying various companies bike geometry charts to see what measurements are largely the same over a range of frame sizes. It seems to me that some of the measurements don't change much, if at all, in the middle of the size range (say 52/54/56/58).
So, with that in mind, can I build a simpler jig that picks a couple of set values for some areas and just goes with those?
I think I will set it up with the chainstay length, bottom bracket drop, and seatpost/head tube angles set. The only adjustment will be the positioning of the head tube for longer/taller frame sizes (leaving the seat post taller is easy).
Workable? Suitable for a budget home project?
Jig design will be based around a pair of parallel steel or al sections with everything working up from there. This pic shows something similar, but doesn't show a fore/aft movement on the head tube. This idea shows how I might use the twin sections to help locate things along a central axis.
I've got an experienced welder (from another industry) who is helping me make it cheap and has provided a lot of insight on to how to make it. He's asked lots of questions and I've had to work out why I wanted to do things
I think the answer to this question depends on what you are going to do with it. A custom frame is not very custom if the geometry can't be tweaked to fit the rider so I'm not in favor of limiting your options. If on the other hand you are only going to build for yourself and maybe a small select group of others I don't see the harm.
Nice link by the way, I love it when people are fabricating stuff instead of just purchasing off the shelf.
Can't afford to buy one! Luckily I can get the welding cheap and my place of work has a metal workshop that can help with engineering some of the parts I need (threaded cones to hold head tube and bottom bracket for eg).
Originally Posted by Nessism
The project is just going to be me and a couple of other people.
First up will be a bamboo singlespeed, then a bamboo geared road bike. The bamboo tubing eliminated many of the possible jig designs - all the ones that grip the tubes mid-way. Bummer really as I liked Cassave's frame jig!
If I build the jig right, it will do the bamboo bikes now, then it will also be suitable for another job doing a brazed or TIG steel frame in the future.
I came across this design recently. It has more than enough adjustability for your needs. It could be made for about $150 less machining.
If you work on the Sabbath your jig will be an abomination.
Originally Posted by JonnyHK
I built this on a series of Sundays.
Originally Posted by Starck
Maybe my God is different to your God?
On the question of limiting the adjustments, I think this is practical mainly from the perspective of someone with a limited number of frame designs in mind. Sachs makes a lot of cyclo cross bikes I believe, and he makes them all with the same BB drop, and he won't compromise. So a person who wanted to make a few cross bikes, might model that input and decide they could get away with a rear unit that fixed chainstay length, and BB drop, probably even seat post angle if they knew their measurements for sure, and that is often the same number anyway. That would leave one moveable part the headtube positioning. I came up with a number of ways to make jigs of this type very simply, though I never did them because I am the opposite type of builder. I build only for myself, and every bike is likely to be different. A cargo bike, a touring bike, a folder, an MTB, a tandem. Just changing wheel diameter or even tire type might lead me to change the BB drop. The simplified jig is useless to me.
Since jigs aren't necesarry or arguably even desireable, and bad jigs aren't particularly useful, I think for many small shops that are not commercial in orientation it is hard to make a case for jigs. The main reason people build them is because they want them. It seems like the main specialist tool for a frame builder. Like the main tool for a cabinetmaker is the workbench.
I have been down the welded jig path, and have been pretty disappointed. The welded jig is very difficult to make accurate, because essentially the problems are the same as framebuilding; it is tough to super heat metal and keep it aligned. The added challenge is that most weld fab steel is grossly inaccurate. If I were thinking of composite frames where the parts don't move under heat, or even require tight tolerances in assembly, epoxy and cloth bridge gaps, it would be pretty hard to argue me out of plywood jigs. It is easy to get into the accuracy range of milled metals, with cheap tools that cut very quickly. I can take a 2x4 edge and make it straight within a thou every 2 feet in a matter of minutes even seconds. Much faster compared to the time it takes to machine anything, that would take me many hours and I don't have the gear to do it on large pieces.
My way to have my cake and eat it too is to use milled metal parts scavenged from machines. They are true and made of stable materials. They are cheap and already pre-machined. Using these parts is still taking a lot of time because even the small amount of machining left over is costly and time consuming.
Last edited by NoReg; 02-08-09 at 12:57 AM.
Ok, I looked at your two pics higher in the thread. The "this idea" example is similar to an originally chopper type jig I made. I chose the Chrime Scene Chopper design because of the free plans, and excellent detailing. One of the interesting features of that design is that he used cold rolled plate to surface the rails to deal with the inaccuracy in the tubes. I tried that and had some trouble so I wrote CSC, and the reply was that it was a bear for them also. Obviously if the tubes aren't straight, then capping them isn't easy either. Essentially two tubes in that arrangement are very similar to a lathe bed. Right at the moment I have a guy offering me a minty Sheldon 10 inch lathe for 200 bucks. That's awesome, though I have 5 lathes already. I think a cheaper and always available wood lathe bed would be a better bet. At those kinds of price, who the heck can afford to built it. The idea the pictured recumbent jig is accurate to a thou or two over 6 feet is ridiculous. Machine tools hold that kind of tolerance if they are really nice, possibly an Anvil jig. Anything else is lucky to be within an 1/8". Now could tubes be accurately held on a junk jig? As the builder says:
"(Note: The jig is a work in progress. The adjustability is not optimum because moving one piece may effect the alignment on another piece making precise setup difficult and time consuming.)"
It can be done, but you almost need a jig to set the jig. I have thought of using a laser machine tool aligner. They are useful anyway, and could be used to take the slop out of a sloppy jig, but even that isn't as easy as it sounds. And what use is a jig that doesn't have it's own accuracy.
Last edited by NoReg; 02-08-09 at 01:27 AM.
Originally Posted by Peterpan1
Dang, $200 sounds like a killer deal! I'd love to hook into something like that. Used to own a well worn Southbend 9" and loved that thing even though it was pretty tired. Going to have to start watching Crackslist more often.
I think you're gonna need a bigger boat.
Originally Posted by JonnyHK
I see things in socal that are awesome compared to what we get. The Sheldon is a special situation, but for better or worse things are showing up right now that seem pretty nice. Still looking for one of these though: